Ion beam processing systems are used in a variety of applications for modifying the properties of a substrate during the fabrication of thin film devices, such as semiconductor and data storage devices. In particular, etching steps may be used to remove and shape layers of material on a substrate. A conventional etching procedure involves the use of a working gas ionized into a plasma state at low pressures (i.e., pressures less than about 1 mTorr) from which ions are extracted and accelerated by ion optics for ion beam etching (IBE) of wafer materials
As device critical dimensions shrink, the need for improved process uniformity without sacrificing beam directionality has driven the search for improved ion sources. IBE uniformity is directly related to the beam current density distribution of ions and energetic neutrals arising from charged ions converted to neutrals in charge exchange ion-atom collisions during the beam transport. The integrated beam particle flux density should be independent of the impact position on the substrate. The angular distribution of the charged and neutral beam particles at the substrate is directly related to the angular properties of the trajectories of the ions extracted from the plasma by the optics of the source. To optimize process uniformity, the incident angle of the particles across the substrate should be approximately parallel.
Conventional ion sources commonly utilize a helical or coil antenna that is wrapped about a discharge vessel to generate an inductively coupled plasma (ICP) using high frequency energy, such as radio-frequency (RF) energy. The antenna of the ion source, when carrying an oscillating high frequency current, induces a time-varying magnetic field inside the discharge vessel. In accordance with Faraday's law, the time-varying magnetic field induces a solenoidal high frequency electric field, which accelerates the electrons in an azimuthal direction in the discharge vessel and sustains the ICP. Because the low pressure ICP is diffusion dominated, the plasma density and, thereby, the radial plasma ion flux density distribution at the ion optics plane, of a conventional broad ion beam source is invariably convex, i.e. highest at the center of the source and decreasing radially. This introduces non-uniformities into the ion current density distribution of the broad ion beams generated by such conventional ion sources.
Typical broad beam ion sources utilize a multi-electrode accelerator system for forming and accelerating the ions into a beam. The electrodes in this system are flat or dished multi-aperture plates, typically called grids. A conventional method of compensating for the effect on the ion beam profile of the plasma non-uniformities described above is to radially vary the transparency of the grids so as to decrease the beam current density in the center. However, this compensation method has several limitations. Variations in the transparency of the ion optics cannot compensate for variations in the plasma density profile for different ion source operating conditions (rf power, beam voltage and current, gas type and pressure), for any time dependence of these factors between system maintenance periods, or for variations in source and ion optics, either short and long term service condition changes in a given etch module because of the effect of mass- and thermal loads or module-to-module variations due to differences in ion source or grid construction. Furthermore, a concave or convex, rather than flat, beam profile is sometimes desirable for a particular process, to compensate for variations in other aspects of processing of the substrates, such as beam spreading during transport to the wafer, clamp effects at the periphery of the substrate, or variations in the thickness of the material layer being etched or the width of the etch mask features.
In addition, localized variations in the plasma radial and azimuthal density distributions are observed, which typically limit the uniformity of the IBE process. The location and shape of these variations depend on the operating conditions and thus are also not easily compensated for by grid optics transparency tuning.
What is needed, therefore, are improved ion sources for generating an energetic beam of particles, such as ions, having enhanced control over the ion current density distribution.